As you observe, I it did seem to be rock solid--I'm using the past tense because I can't resist making it a little bit more so, and making it easier to orient the camera vertically. So right now the assembly is back in the hands of my inimitable fabricator, Don Wilson. We are adding two more ring mounts, a length of right-angle aluminum stock, and a second Arca-Swiss plate. Will of course share information after this alteration is completed.

Since posting this thread, I've upgraded the Bratcam's tube lens assembly. For anyone considering something similar, I recommend incorporating these additonal features. Both the earlier and current embodiments produce good results, but the newer version makes repeatable results easier to obtain. The rig now has a more confident feel--more support, and a pleasing additional level of solidity; it now feels as solid as a crowbar. I can change lenses or add the polarizing adapter with not the slightest change in framing, even at high magnification. And now I can switch between horizontal and vertical compositions very easily.

Seen from the side--camera goes at the right end, Mitutoyo objective at the left end.

H: Another Arca-Swiss rail from Chris Hejnar (not an Edmund part—about $40). There are now two of these in the assembly. The other one will be shown in the next image.

I: Another T-Mount 48mm Ring Mount with ¼-20 tapped hole: (Edmund stock number NT52-304, $65). There are three of these in the rig. This one grips the focusable T-tube, and adds substantial rigidity.